The present invention relates to an optical fiber coupling system for optically coupling an optical fiber and another optical element to each other in the field of optical communication and particularly to suppression of back reflection of light from an end surface of an optical fiber.
In the field of optical communication, light propagated through an optical fiber and light input/output to/from any kind of optical element such as a photodetector or a light source need to be coupled to each other. On this occasion, back reflection of light from an end surface of the optical fiber to the light source (semiconductor laser) side has large influence on stability of signal transmission. Hence, such back reflection needs to be suppressed to be extremely small.
The back reflectance BR is expressed by the ratio of the intensity PBR of back reflection of light to the intensity Pin of incident light.BR[dB]=10 log(PBR/Pin)
When an ordinary silica optical fiber is cut by a fiber cutter or the like, the back reflectance BR of a flat end surface of the cut optical fiber is about −14 dB which is a very large value compared with the required value (−40 dB or smaller) in the field of optical communication. Therefore, methods such as a method having the steps of: obliquely polishing an end surface of an optical fiber; and forming an anti-reflection film on the end surface have been proposed as measures to reduce back reflection of light (e.g. Japanese Patent Laid-Open No. 2001-21775).
In various kinds of optical modules, means for filling an end surface of an optical fiber with a resin or the like to perform refractive index matching has been also used when an optical element and the optical fiber are coupled to each other. When light is incident from a medium with a refractive index n1 onto a medium with a refractive index n2 perpendicularly to the interface between these two media, the reflectance R is expressed by the following equation.R={(n1−n2)/(n1+n2)}2
Hence, when light is emitted from a silica optical fiber having a core with a refractive index n1=1.46 into air, such reflection occurs. Therefore, in most cases, the forward end of the optical fiber is generally bonded/fixed by a translucent resin with a refractive index substantially equal to 1.46 to be matched with the refractive index of the core of the silica optical fiber.
Productivity is however low because complicated steps are required for obliquely polishing the end surface of the optical fiber and forming the anti-reflection film on the end surface. Particularly in a multi-core tape fiber which has been used widely with the recent increase in communication capacity, there is a problem that it is difficult to produce the multi-core tape fiber.
On the other hand, it is said that the refractive index of the refractive index-matching resin changes from about 1.37 to about 1.58 when the temperature in use changes from −40° C. to +85° C. (as disclosed in Japanese Patent Laid-Open No. 2001-21775). The refractive index of the refractive index-matching resin depends extremely largely on the temperature. Generally, the refractive index of resin as well as the refractive index-matching resin has a temperature coefficient of about 10−3° C.−1. The temperature coefficient of the refractive index of silica is negligibly small compared with the temperature coefficient of the refractive index of resin. Hence, even in the case where the refractive index of the optical fiber and the refractive index of the resin are matched with each other at room temperature, there is a possibility that reflection may increase to be higher than a tolerance limit in the vicinity of the upper or lower limit of the temperature in use because of the refractive index difference caused by the change of the ambient temperature.